Compaction and Subsidence Geomechanics: A Case Study from the North Sea

Abstract We present a case study for the assessment of depletion of the Blythe Field in the Southern North Sea and the potential impact on the overlying Dudgeon windfarm, including an analytical solution, and make comparisons with the well-studied analogue Groningen Field before presenting the results. The key focus of this analysis was to aid in the assessment of how oil and gas activities could co-exist with other operations in the same location, namely the impact between infrastructures at the surface and in the sub-surface. The results of the study, and the validation via the comparison to the analogue Groningen Field, provide confidence that production and depletion of the Blythe Field will have minimal-to-no impact on the Dudgeon windfarm that sits directly above the field on the seabed. Studies of this type allow for conventional oil and gas activities and other types of energy production to be safely risked and assessed in terms of being active in the same location, maximizing the potential for energy generation whilst minimizing the risk to the infrastructure, the environment, and to health and safety. Introduction Blythe and Elgood were initially discovered between 1966 and 1990 and now sit under the Dudgeon windfarm. The current operator plans to drill two wells to produce 70 bcf of gas during 6 years of production, with an associated pore pressure decrease and reservoir compaction which may affect the overburden sequence and requires assessment. Reservoir compaction and land subsidence occur when large amounts of pore fluids have been withdrawn from underground reservoirs. Magnitudes of compaction and subsidence in oil and gas fields vary greatly depending on characteristics of the rock properties and field geometry. The effects of compaction and subsidence range from beneficial to detrimental; examples include seabed subsidence at the Ekofisk Field leading to remedial action required on the jack-up rig and casing damage such as that at the Valhall and Shearwater fields. Therefore, assessment of the potential for large magnitude compaction and subsidence must be carried out during field planning.